Industrial heat treat furnaces are conventionally designed for the particular heat treat process which is to be accomplished by the furnace. Obviously, a furnace developed for heat treat processes requiring temperatures in excess of 2000.degree. F. requires different heat transfer considerations than a furnace designed to heat the work at temperature ranges of approximately 1000.degree. F. Also, should the process temperature be further reduced to approximately 500.degree. F., ovens using panel construction are used in place of furnaces.
There are a number of industrial applications where metal parts must be tempered after quenching and it is simply not economically feasible to effect tempering in high temperature, heat treat furnaces. Alternatively, the customer may desire to temper the work himself. Such considerations have resulted in a market for draw or temper furnaces typically operating at temperatures of about 1250.degree. F. or at about 800.degree. F. At such temperatures, heat transfer with the work is principally achieved by convection. Traditionally, convection is achieved by simply mounting fans in box type furnaces which use a baffle arrangement to cause circulation of the heated atmosphere or wind with the work. The market for tempering furnaces obviously represents the low price end of the heat treat furnace market and is intensely cost-competitive.
For several years now, metallurgical process requirements have been consistently tightened to require closer control of the temperature uniformity in the work to produce higher quality parts. It is not uncommon for a customer to require a total heat treat temperature spread of no more than 10.degree. F. (i.e. .+-.5.degree. F.). A furnace designer confronted with such requirement must first design the furnace to achieve temperature uniformity at any point within the furnace enclosure without a load present. Only after temperature uniformity has been achieved in the furnace design does the focus next turn to the process time-temperature requirements for the work, i.e. heat transfer rate. As appreciated by those skilled in the art, any number of factors can result in a heat sink, heat source or hot spots produced within the furnace enclosure which prevents achievement of the desired temperature uniformity. The temperature uniformity problem is further complicated because the heat transfer medium itself can produce temperature deviations such as heat transfer by radiation conflicting with heat transfer from convection, etc.
A number of furnace designers believe that the traditional box furnace configuration is not conductive to achieving uniform temperature distribution within the temperature ranges required in today's market. Accordingly, positive pressure, batch type cylindrical furnaces have been developed in the belief that such furnaces inherently will eliminate hot spots or heat sinks when compared to the box furnace. Again, the underlying premise is that if the furnace temperature can be maintained within the temperature uniformity requirements anywhere within the furnace enclosure, then in time the work temperature will homogenize itself to that of the furnace temperature.
As a practical matter however, economic considerations dictate, at least with respect to operating temper furnaces, that each batch be processed in as quick a time as possible. This is traditionally accomplished by means of baffles, distribution plates, dampers and/or nozzles which direct the heated, furnace atmosphere or wind against the work. An example of such an arrangement is shown in FIG. 1 which illustrates a commercially successful, prior art cylindrical temper furnace developed by the assignee of the present invention. In the cross-sectional schematic of FIG. 1, the work "W" is shielded on three sides by a housing "H" connected to a fan plenum "P". A baffle "B" and adjustable dampers "D" insure an atmosphere flow about work "W" to effect uniform convective heat transfer within a satisfactory temperature range. Note that fan "F" is typically mounted through the cylindrical furnace casing. While the temper furnace disclosed in FIG. 1 does meet temperature uniformity requirements, nevertheless the fan mounting, the baffling and housing increases the furnace cost. Also, the pressure of such structure inherently effects temperature uniformity. In addition, the fact that the dampers must be adjusted sensitizes, somewhat, the furnace operation although perhaps no more than that of the other prior art arrangements. The present invention is an improvement over the FIG. 1 prior art furnace.
The prior art thus far described, relates to batch type, positive pressure furnaces. There are, of course, vacuum furnaces in widespread conventional use in the heat treat field. Vacuum furnaces and variations thereof (such as ion nitriders) are double walled pressure vessels, and are typically formed as cylinders with spherical ends. It is to be appreciated that box type furnaces represent a configuration which cannot economically function as a pressure vessel. In a vacuum furnace, the work is heated and while under a vacuum, a treatment gas is backfilled into the chamber to impart the desired case properties into the work. The process cycle usually requires the work to be quenched after heating. A number of recent developments have been made in vacuum furnaces to permit the work to be rapidly gas quenched. The quench schemes use special nozzle distribution plates, baffles, dampers and the like, all of which are designed to blast the work with high speed gas jets. The concept is to impinge the entire surface of the work with turbulent gas jets to achieve a heat transfer rate which approximates a liquid quench. U.S. Pat. No. 4,836,766 to Jomain (incorporated herein by reference) illustrates a typical approach where baffling in combination with a high speed helical jet is used to spray the work in a gas quench. Traditionally, a liquid quench is effected in a separate chamber of the furnace at atmosphere pressure.
There are numerous, convective heat transfer arrangements in the prior art and it is known to use the intake of a fan as a centrally positioned under pressure zone to cause recirculation of furnace atmosphere. This is shown, for example, in the baffled arrangement of the Jomain patent. There are variations. In U.S. Pat. No. 4,789,333 assigned to Gas Research Institute (incorporated herein by reference) a free-standing circular jet is developed through an orifice and expanded into turbulent contact with a cylindrical shell member as the jet travels the length of the cylindrical shell. At the end of the shell, the jet is redirected by a special diverter plate to impinge the work and the spent jet is then collected through the under pressure zone to be recirculated. While such an arrangement appears satisfactory to effect high temperature heat transfer with a thin shell, the turbulence caused by the jet would have a deleterious effect on the insulation in a temper furnace. U.S. Pat. No. 4,395,233 to Smith et al (incorporated herein by reference) also illustrates the use of a central under pressure zone to cause recirculation of forced air in a baking oven. However, Smith's oven is rectilinear in configuration and this will cause turbulence at the oven corners, and while this may be acceptable at the relatively low pressures in an oven application, such an arrangement is unacceptable at the high mass flow rates required in furnace applications. None of the arrangements is sufficient to develop the "wind" pattern required in the heat treat furnace applications to which the present invention is concerned.